Low molecular weight polytetrafluoroethylene (PTFE) powders are widely used as additives to other materials such as inks, coatings, greases, lubricants, and plastics. These powders are currently produced either by direct polymerization or by degradation of high molecular weight PTFE powders through either high energy irradiation or thermal processes.
Direct polymerization has generally been accomplished by “dispersion” polymerization (also known as “emulsion” polymerization) wherein the polymer is produced as sub-micrometer colloidal particles, i.e., particles having a size less than 1 micrometer, in an aqueous latex. Dispersion polymerization of fluoropolymers often, but not always, employs a fluorinated surfactant for stabilization of the dispersion during polymerization. Dispersion polymerization can sometimes be carried out without surfactants if agitation is gentle and polymer solids are low (i.e., less than 20%). For example, Japanese patent publication (Kokoku) 57-22043 to Fumoto et al., discloses the preparation of low molecular weight PTFE with or without surfactant but calls for high levels of fluorine-containing telogen (fluoro- or chlorofluoro-alkanes) and achieves only low solids levels. In dispersion polymerization processes, the product latexes are coagulated subsequent to polymerization to afford agglomerated powders. The surfactants used to stabilize the dispersions are normally removed during isolation/drying. The powders are generally rather friable and may be deagglomerated upon application of shear back toward the original sub-micrometer primary particles. Such deagglomeration is especially prevalent if the powders are dispersed into a liquid having a low surface tension. These powders also typically have high surface areas such as 8–20 m2/g.
The particle size of the low molecular weight PTFE powders is very important for some of its applications as an additive. For example, the preferred particle size for a coating or an ink print is typically slightly larger than the thickness of the coating or ink print. PTFE particles that protrude slightly through the coating layer increase the smudge or scuff resistance of printed inks. Such coatings or ink layers are typically 2–40 micrometers in thickness. Sub-micrometer particles such as those obtained from prior-art dispersion direct polymerization are too small to protrude through the coating or ink surface and cannot significantly aid in smudge resistance.
The high surface area of low molecular weight PTFE additive powders can increase the viscosity of the formulations into which it may be added. Although a high thickening powder is sometimes desirable, a minimal effect on formulation viscosity is at other times wanted. Again the high surface areas of the prior-art dispersion polymers may be a disadvantage.
Low molecular weight PTFE powders for uses, wherein a 2–40 micrometer particle size and/or a low surface area is preferred, have been produced typically from high molecular weight “suspension” polymerized PTFE (usually referred to as “granular” PTFE) by thermal degradation or by irradiation with high energy electrons from either a gamma source or electron beam. Granular PTFE can, after degradation of its molecular weight, be ground to the desired particle size for use in coatings and inks. Such powders typically have a much lower surface area (1.0–4.0 m2/g) than polymer from emulsion or dispersion polymerization. However, these degradation processes are expensive and also generate hazardous byproducts, such as hydrogen fluoride.
It has also been attempted to change the particulate nature of low molecular weight dispersion polymerized PTFE powders to be more like powders produced by irradiating or thermally degrading high molecular weight PTFE. In U.S. Pat. No. 5,118,788 to Hosokawa et al., unsintered low molecular weight dispersion polymerized PTFE powder is heated to a temperature in range from about 70° C. lower than the melting point to a temperature lower than the melting point and comminuted. Such a process, however, is inherently difficult or impossible to control. As illustrated in the comparative examples of U.S. Pat. No. 5,118,788, a high temperature or long heating time will lead to poor comminuting properties and a low temperature or short heating time will not fuse all of the agglomerates and specific surface area will not be decreased. The powder isolated from a dispersion polymerization has a variety of agglomerate sizes and tightness of packing of the primary particles. This variation in the powder characteristics will cause variable agglomerate fusing at temperatures below the resin melting point. Incomplete fusing leads to the presence of friable agglomerates of primary dispersion particles, these agglomerates may deagglomerate to particles of less than 1 micrometer in size upon further processing such as mixing, grinding and the like.
A direct polymerization process for efficient production of low molecular weight PTFE powders suitable for use as additives to other materials is desired that can (1) minimize or eliminate the use of fluorosurfactants which are expensive and present a disposal problem after polymerization; (2) provide a powder substantially free of friable agglomerates of sub-micrometer primary particles; (3) provide a powder with a surface area less than 8 m2/g; and/or (4) minimize or eliminate the need for polymer degradation processes.